To improve data transmission efficiency in a WLAN (wireless local area network), a 4× symbol length is introduced into a next-generation WLAN standard 802.11ax, and correspondingly, an 802.11a/n/ac symbol is referred to as a 1× symbol.
The 4× symbol length means that a data length is 12.8 μs in an OFDM (orthogonal frequency division multiplexing) symbol. Correspondingly, a proportion that a CP of 3.2 us accounts for in an OFDM symbol changes to (3.2/(3.2+12.8))=20%, so that transmission efficiency is effectively improved. It may be learned that a time domain transmission time of the data part changes from 3.2 us to 12.8 us and is increased by four times. Correspondingly, in a frequency domain, bandwidth of each subcarrier is decreased by four times because smaller bandwidth indicates a longer transmission time. Specifically, for 802.11ac, there are 64 subcarriers on a bandwidth of 20 MHz, and the 64 subcarriers are corresponding to 64-point FFT; there are 128 subcarriers on a bandwidth of 40 MHz, and the 128 subcarriers are corresponding to 128-point FFT; there are 256 subcarriers on a bandwidth of 80 MHz, and the 256 subcarriers are corresponding to 256-point FFT. For 802.11ax, there are 256 subcarriers on 20 MHz, and the 256 subcarriers are corresponding to 256-point FFT; there are 512 subcarriers on 40 MHz, and the 512 subcarriers are corresponding to 512-point FFT; there are 1024 subcarriers on 80 MHz, and the 1024 subcarriers are corresponding to 1024-point FFT.
A bandwidth of 20 MHz is used as an example, the 64 subcarriers in 802.11ac include 52 data subcarriers and four pilot subcarriers, and the 256 subcarriers in 802.11ax include 234 data subcarriers and eight pilot subcarriers. If a same MCS (Modulation and Coding Scheme) is used, a volume of data that can be transmitted in 802.11ax may be greater than four times that can be transmitted in 802.11ac, because (234>4×52). It is the same for 40 MHz and 80 MHz.
After the 4× symbol length is introduced, a time required for a receive end to process each OFDM symbol increases. A processing process at the receive end mainly includes: 1. FFT Fast Fourier Transform; 2. demapping; and 3. channel decoding. The most time consuming part is channel decoding. The time for channel decoding increases because the data volume in each OFDM symbol increases. This processing delay becomes very severe in a case of large bandwidth (80 MHz or the like) and a high MCS (for example, MCS9).
When receiving a data frame or a control frame that requires an immediate response (a response after a SIFS=16 us), the receive end needs to first process the data frame or the control frame, and then switch from a receiving state to a sending state. These two parts need to be completed within a SIFS (Short Interframe Space) time. For a 1× symbol length (that is, an 802.11a/n/ac frame), SIFS duration of 16 us is sufficient for the receive end to complete data processing and state switching. However, for a 4× symbol length(that is, an 802.11ax frame), data processing may cause a relatively long delay. Consequently, in a case of current SIFS duration of 16 us, the receive end cannot complete data processing and state switching.